Friends and colleagues of Sen. Edward M. Kennedy instinctively reached for the same word Tuesday as they learned that the last remaining representative of Camelot had been diagnosed with a malignant brain tumor.

They called him a fighter. They said he had fought for children, the poor, African-Americans and immigrants during his nearly 46 years in the Senate. He fought for access to health care, for quality education, for civil rights and for workers' rights. And now, they said, he would fight for himself.

"I know that fighting spirit will hold him in good stead in the challenge that he faces now," said House Speaker Nancy Pelosi (D-Calif.).

"Those of us who've stood by his side know that there is no better ally and no more determined fighter," Sen. Dick Durbin (D-Ill.), the assistant Senate majority leader, said on the Senate floor. "Now, as Ted Kennedy faces another great fight, we know that he will bring the same courage to the battle."

President George W. Bush, as well as all three presidential candidates, paused to pay tribute to the Massachusetts Democrat's legislative achievements.

Sen. Barack Obama (D-Ill.), who received Kennedy's backing for the Democratic presidential nomination, said he might not be a member of the Senate had it not been for Kennedy's work on voting rights and civil rights. Sen. Hillary Clinton (D-N.Y.) called him one of the greatest legislators in Senate history.

And Sen. John McCain (R-Ariz.), Kennedy's chief co-sponsor on an unsuccessful overhaul of immigration laws, blinked back tears on his campaign bus as he called Kennedy "the last lion in the Senate."

"I have held that view because he remains the single most effective member of the Senate if you want to get results," McCain said. "He is not reluctant to share the credit, and when it fails he's willing to take the blame."

A seizure Saturday morning at his Hyannis Port home sent Kennedy to Cape Cod Hospital before being flown by helicopter to Massachusetts General Hospital in Boston.

Tests over the weekend identified the cause of the seizure as a malignant glioma in the left parietal lobe, his doctors said Tuesday in a statement. Kennedy will stay in the hospital for the next couple of days for further tests, the physicians said, noting he "remains in good spirits and full of energy."

Kennedy was elected to the Senate in 1962 at age 30 to take the place of his brother, John F. Kennedy, who had become president. Now 76, he is the third-longest-serving senator in history, behind Sen. Robert Byrd (D-W.Va.) and the late Sen. Strom Thurmond of South Carolina.

The youngest of nine children, Kennedy comes from a storied political family that has captured the public's imagination in a way unequaled by any other political dynasty. But it is also a family that has been struck repeatedly by tragedy. Kennedy's oldest brother, Joseph, was killed during World War II in a plane crash. President Kennedy was assassinated in Dallas in 1963. And Robert, a U.S. senator from New York, was shot and killed in Los Angeles in 1968 while running for president.

Kennedy himself ran for president in 1980, challenging President Jimmy Carter for the Democratic nomination. He was never able to clearly articulate why he was running and he was also peppered anew with questions about the death of Mary Jo Kopechne, a passenger in his car during a July 1969 car accident at Chappaquiddick Island, Mass., where she drowned.

At the Democratic National Convention in 1980, as his quest for the presidency came to a close, Kennedy left the public with words that continue to resonate, saying, "For all those whose cares have been our concern, the work goes on, the cause endures, the hope still lives, and the dream shall never die."

Though he never achieved the presidency, Kennedy is without question a master of the Senate.

"This is the most consequential legislative career in the country's history," said Thomas Oliphant, who chronicled Kennedy's career as a correspondent and columnist for The Boston Globe. "It probably had more impact on more people than many presidents. What's so remarkable about it is not only that it's been so consequential, but it's been that way for so long."

Kennedy arrived in the Senate at a time when lawmakers of both parties worked together and socialized together, forming bonds of friendship that allowed them to overcome partisan differences.

"Except for a couple minimum wage increases, there isn't a single legislative accomplishment of his that was done without the help of a Republican partner," said Adam Clymer, Kennedy's biographer.

Any number of laws bear his name. With former Sen. Nancy Kassebaum (R-Kan.), Kennedy passed legislation to require that health insurance be portable for workers who move from one employer to another. With Sen. Orrin Hatch (R-Utah), Kennedy helped to create the SCHIP program, providing health coverage to children from low-income families.

With former Senate Majority Leader Bob Dole (R-Kan.), Kennedy helped to pass the Americans with Disabilities Act as well as the Civil Rights Restoration Act, which requires recipients of federal funds to comply with civil rights laws.

Kennedy also has been a force in foreign affairs. He led the effort to impose sanctions on South Africa for its apartheid system. He was responsible for getting the Senate to cut off arms sales to Augusto Pinochet, the repressive president of Chile. And he pressed hard for peace in Northern Ireland.

On the Senate floor, Kennedy is known for his lengthy and voluble stem-winders, with his ruddy face getting redder by the minute and his voice thundering through the chamber.

Even while many Republicans were personally friendly with him, they have benefited from his larger-than-life presence, raising millions of dollars with fundraising letters urging people to help fight the liberal giant.

His colleagues say he has always carried a heavier legislative load than any other senator. And he has done so often stooped over and wracked with pain, due to a 1964 plane crash that broke his back.

"Everyone I know just marvels at his strength," Durbin said. "Any other person would have been classified as a disabled person and unable to work after the injuries he sustained. He's never complained, but I've seen the pain on his face. He soldiers on without a word of complaint."